Descent motions of the Huygens probe as measured by the Surface Science Package (SSP): Turbulent evidence for a cloud layer

The Huygens probe underwent vigorous short-period motions during its parachute descent through the atmosphere of Saturn's moon Titan in January 2005, at least some of which were excited by the Titan environment. Several sensors in the Huygens Surface Science Package (SSP) detect these motions, indicating the transition to the smaller stabilizer parachute, the changing probe spin rate, aerodynamic buffeting, and pendulum motions. Notably, in an altitude range of about 20–30 km where methane drops will freeze, the frequency content and statistical kurtosis of the tilt data indicate excitation by turbulent air motions like those observed in freezing clouds on Earth, supporting the suggestion of Tokano et al. [Tokano, T., McKay, C.P., Neubauer, F.M., Atreya, S.K., Ferri, F., Fulchignoni, M., Niemann, H.B. (2006a). Methane drizzle on Titan. Nature 442, 432–435] that the probe passed through such a cloud layer. Motions are weak below 20 km, suggesting a quiescent lower atmosphere with turbulent fluctuations of nominally <0.15 m/s (to within a factor of ∼2) but more violent motions in the upper troposphere may have been excited by turbulent winds with amplitudes of 1–2 m/s. Descent in part of the stratosphere (150–120 km) was smooth despite strong ambient wind (∼100 m/s), but known anomalies in the probe spin prevent investigation of turbulence in the known wind-shear layer from 60 to 100 km.     

Dynamical properties of the Venus mesosphere from the radio-occultation experiment VeRa onboard Venus Express

The dynamics of Venus’ mesosphere (60–100 km altitude) was investigated using data acquired by the radio-occultation experiment VeRa on board Venus Express. VeRa provides vertical profiles of density, temperature and pressure between 40 and 90 km of altitude with a vertical resolution of few hundred meters of both the Northern and Southern hemisphere. Pressure and temperature vertical profiles were used to derive zonal winds by applying an approximation of the Navier–Stokes equation, the cyclostrophic balance, which applies well on slowly rotating planets with fast zonal winds, like Venus and Titan. The main features of the retrieved winds are a midlatitude jet with a maximum speed up to 140 ± 15 m s^?1 which extends between 20°S and 50°S latitude at 70 km altitude and a decrease of wind speed with increasing height above the jet. Cyclostrophic winds show satisfactory agreement with the cloud-tracked winds derived from the Venus Monitoring Camera (VMC/VEx) UV images, although a disagreement is observed at the equator and near the pole due to the breakdown of the cyclostrophic approximation. Knowledge of both temperature and wind fields allowed us to study the stability of the atmosphere with respect to convection and turbulence. The Richardson number Ri was evaluated from zonal field of measured temperatures and thermal winds. The atmosphere is characterised by a low value of Richardson number from ～45 km up to ～60 km altitude at all latitudes that corresponds to the lower and middle cloud layer indicating an almost adiabatic atmosphere. A high value of Richardson number was found in the region of the midlatitude jet indicating a highly stable atmosphere. The necessary condition for barotropic instability was verified: it is satisfied on the poleward side of the midlatitude jet, indicating the possible presence of wave instability.     

Electron conductivity and density profiles derived from the mutual impedance probe measurements performed during the descent of Huygens through the atmosphere of Titan

During the descent of the Huygens probe through the atmosphere of Titan, on January 14th, 2005, the permittivity, waves and altimetry (PWA) subsystem, a component of the Huygens atmospheric structure instrument (HASI), detected an ionized layer at altitudes around 63 km with two different instruments, the relaxation probe (RP) and the mutual impedance probe (MIP). A very detailed analysis of both data sets is required, in order to correct for environmental effects and compare the two independent estimates of the electrical conductivity. The present work is dedicated to the MIP data analysis. New laboratory tests have been performed to validate or improve the available calibration results. Temperature effects have been included and numerical models of the MIP sensors and electric circuitry have been developed to take into account the proximity of the Huygens probe body. The effect of the vertical motion of the vessel in the ionized atmosphere is estimated in both analytical and numerical ways. The peculiar performance of the instrument in the altitude range 100–140 km is scrutinized. The existence of a prominent ionized layer, and of enhancements in the conductivity and electron density profiles at 63 km, are discussed in the light of previous theoretical predictions.     

Characteristics of planetary-scale waves simulated by a new venusian mesosphere and thermosphere general circulation model

We have developed a new general circulation model (GCM) for the venusian mesosphere and thermosphere (80-about 180 km). Our GCM simulations show that winds in the subsolar-to-antisolar direction (SS–AS) are predominant above about 90 km. A weak return flow of the SS–AS is seen below about 90 km. We performed GCM simulations imposing the planetary-scale waves (thermal tides, Rossby wave, and Kelvin wave) at the lower boundary. Although the diurnal and semidiurnal tides are damped below 95 km, the Rossby wave propagates up to around 130 km. However, the amplitude of the Rossby wave is too small (<1 m/s) to affect the general circulation. On the other hand, the Kelvin wave propagates up to about 130 km with a maximum zonal wind fluctuation of approximately 5.9 m/s on average. The amplitude of the Kelvin wave sometimes exceeds 10 m/s around the terminator. The Kelvin wave causes a temporal variation in the wind velocity at the altitude of the O₂-1.27 μm nightglow emission (about 95 km). Using a newly developed 1-D nightglow model and the composition distribution calculated from our GCM, we investigated the impact of the Kelvin wave on the nightglow distribution. Our results suggest that the Kelvin wave would cause temporal variations in the nightglow emission in the 23:50–00:20 LT region with an intensity of 1.1–1.3 MR and a period of approximately 4 days.     

Comparisons of Cassini flybys of the Titan magnetospheric interaction with an MHD model: Evidence for organized behavior at high altitudes

Recent papers suggest the significant variability of conditions in Saturn’s magnetosphere at the orbit of Titan. Because of this variability, it was expected that models would generally have a difficult time regularly comparing to data from the Titan flybys. However, we find that in contrast to this expectation, it appears that there is underlying organization of the interaction features roughly above ～1800 km (1.7 Rt) altitude by the average external field due to Saturn’s dipole moment. In this study, we analyze Cassini’s plasma and magnetic field data collected at 9 Titan encounters during which the external field is close to the ideal southward direction and compare these observations to the results from a 2-fluid (1 ion, 1 electron) 7-species MHD model simulations obtained under noon SLT conditions. Our comparative analysis shows that under noon SLT conditions the Titan plasma interaction can be viewed in two layers: an outer layer between 6400 and 1800 km where interaction features observed in the magnetic field are in basic agreement with a purely southward external field interaction and an inner layer below 1800 km where the magnetic field measurements show strong variations and deviate from the model predictions. Thus the basic features inferred from the Voyager 1 flyby seem to be generally present above ～1800 km in spite of the ongoing external variations from SLT excursions, time variability and magnetospheric current systems as long as a significant southward external field component is present. At around ～1800 km kinetic effects (such as mass loading and heavy ion pickup) and below 1800 km ionospheric effects (such as drag of ionospheric plasma due to coupling with neutral winds and/or magnetic memory of Titan’s ionosphere) complicate what is observed.     

Stratospheric superrotation in the TitanWRF model

TitanWRF general circulation model simulations performed without sub-grid-scale horizontal diffusion of momentum produce roughly the observed amount of superrotation in Titan’s stratosphere. We compare these results to Cassini-Huygens measurements of Titan’s winds and temperatures, and predict temperature and winds at future seasons. We use angular momentum and transformed Eulerian mean diagnostics to show that equatorial superrotation is generated during episodic angular momentum ‘transfer events’ during model spin-up, and maintained by similar (yet shorter) events once the model has reached steady state. We then use wave and barotropic instability analysis to suggest that these transfer events are produced by barotropic waves, generated at low latitudes then propagating poleward through a critical layer, thus accelerating low latitudes while decelerating the mid-to-high latitude jet in the late fall through early spring hemisphere. Finally, we identify the dominant waves responsible for the transfers of angular momentum close to northern winter solstice during spin-up and at steady state. Problems with our simulations include peak latitudinal temperature gradients and zonal winds occurring ∼60 km lower than observed by Cassini CIRS, and no reduction in zonal wind speed around 80 km, as was observed by Huygens. While the latter may have been due to transient effects (e.g. gravity waves), the former suggests that our low (∼420 km) model top is adversely affecting the circulation near the jet peak, and/or that we require active haze transport in order to correctly model heating rates and thus the circulation. Future work will include running the model with a higher top, and including advection of a haze particle size distribution.     

Radiative transfer analyses of Titan’s tropical atmosphere

Titan’s optical and near-IR spectra result primarily from the scattering of sunlight by haze and its absorption by methane. With a column abundance of 92 km amagat (11 times that of Earth), Titan’s atmosphere is optically thick and only ～10% of the incident solar radiation reaches the surface, compared to 57% on Earth. Such a formidable atmosphere obstructs investigations of the moon’s lower troposphere and surface, which are highly sensitive to the radiative transfer treatment of methane absorption and haze scattering. The absorption and scattering characteristics of Titan’s atmosphere have been constrained by the Huygens Probe Descent Imager/Spectral Radiometer (DISR) experiment for conditions at the probe landing site (Tomasko, M.G., Bézard, B., Doose, L., Engel, S., Karkoschka, E. [2008a]. Planet. Space Sci. 56, 624–247; Tomasko, M.G. et al. [2008b]. Planet. Space Sci. 56, 669–707). Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) data indicate that the rest of the atmosphere (except for the polar regions) can be understood with small perturbations in the high haze structure determined at the landing site (Penteado, P.F., Griffith, C.A., Tomasko, M.G., Engel, S., See, C., Doose, L., Baines, K.H., Brown, R.H., Buratti, B.J., Clark, R., Nicholson, P., Sotin, C. [2010]. Icarus 206, 352–365). However the in situ measurements were analyzed with a doubling and adding radiative transfer calculation that differs considerably from the discrete ordinates codes used to interpret remote data from Cassini and ground-based measurements. In addition, the calibration of the VIMS data with respect to the DISR data has not yet been tested. Here, VIMS data of the probe landing site are analyzed with the DISR radiative transfer method and the faster discrete ordinates radiative transfer calculation; both models are consistent (to within 0.3%) and reproduce the scattering and absorption characteristics derived from in situ measurements. Constraints on the atmospheric opacity at wavelengths outside those measured by DISR, that is from 1.6 to 5.0 μm, are derived using clouds as diffuse reflectors in order to derive Titan’s surface albedo to within a few percent error and cloud altitudes to within 5 km error. VIMS spectra of Titan at 2.6–3.2 μm indicate not only spectral features due to CH₄ and CH₃D (Rannou, P., Cours, T., Le Mouélic, S., Rodriguez, S., Sotin, C., Drossart, P., Brown, R. [2010]. Icarus 208, 850–867), but also a fairly uniform absorption of unknown source, equivalent to the effects of a darkening of the haze to a single scattering albedo of 0.63 ± 0.05. Titan’s 4.8 μm spectrum point to a haze optical depth of 0.2 at that wavelength. Cloud spectra at 2 μm indicate that the far wings of the Voigt profile extend 460 cm^?1 from methane line centers. This paper releases the doubling and adding radiative transfer code developed by the DISR team, so that future studies of Titan’s atmosphere and surface are consistent with the findings by the Huygens Probe. We derive the surface albedo at eight spectral regions of the 8 × 12 km² area surrounding the Huygens landing site. Within the 0.4–1.6 μm spectral region our surface albedos match DISR measurements, indicating that DISR and VIMS measurements are consistently calibrated. These values together with albedos at longer 1.9–5.0 μm wavelengths, not sampled by DISR, resemble a dark version of the spectrum of Ganymede’s icy leading hemisphere. The eight surface albedos of the landing site are consistent with, but not deterministic of, exposed water ice with dark impurities.     

Titan’s aerosol and stratospheric ice opacities between 18 and 500 μm: Vertical and spectral characteristics from Cassini CIRS

Vertical distributions and spectral characteristics of Titan’s photochemical aerosol and stratospheric ices are determined between 20 and 560 cm^?1 (500–18 μm) from the Cassini Composite Infrared Spectrometer (CIRS). Results are obtained for latitudes of 15°N, 15°S, and 58°S, where accurate temperature profiles can be independently determined.In addition, estimates of aerosol and ice abundances at 62°N relative to those at 15°S are derived. Aerosol abundances are comparable at the two latitudes, but stratospheric ices are ～3 times more abundant at 62°N than at 15°S. Generally, nitrile ice clouds (probably HCN and HC₃N), as inferred from a composite emission feature at ～160 cm^?1, appear to be located over a narrow altitude range in the stratosphere centered at ～90 km. Although most abundant at high northern latitudes, these nitrile ice clouds extend down through low latitudes and into mid southern latitudes, at least as far as 58°S.There is some evidence of a second ice cloud layer at ～60 km altitude at 58°S associated with an emission feature at ～80 cm^?1. We speculate that the identify of this cloud may be due to C₂H₆ ice, which in the vapor phase is the most abundant hydrocarbon (next to CH₄) in the stratosphere of Titan.Unlike the highly restricted range of altitudes (50–100 km) associated with organic condensate clouds, Titan’s photochemical aerosol appears to be well-mixed from the surface to the top of the stratosphere near an altitude of 300 km, and the spectral shape does not appear to change between 15°N and 58°S latitude. The ratio of aerosol-to-gas scale heights range from 1.3–2.4 at about 160 km to 1.1–1.4 at 300 km, although there is considerable variability with latitude. The aerosol exhibits a very broad emission feature peaking at ～140 cm^?1. Due to its extreme breadth and low wavenumber, we speculate that this feature may be caused by low-energy vibrations of two-dimensional lattice structures of large molecules. Examples of such molecules include polycyclic aromatic hydrocarbons (PAHs) and nitrogenated aromatics.Finally, volume extinction coefficients Nχ_E derived from 15°S CIRS data at a wavelength of λ = 62.5 μm are compared with those derived from the 10°S Huygens Descent Imager/Spectral Radiometer (DISR) data at 1.583 μm. This comparison yields volume extinction coefficient ratios Nχ_E(1.583 μm)/Nχ_E(62.5 μm) of roughly 70 and 20, respectively, for Titan’s aerosol and stratospheric ices. The inferred particle cross-section ratios χ_E(1.583 μm)/χ_E(62.5 μm) appear to be consistent with sub-micron size aerosol particles, and effective radii of only a few microns for stratospheric ice cloud particles.     

Far-infrared opacity sources in Titan’s troposphere reconsidered

We use Cassini far-infrared limb and nadir spectra, together with recent Huygens results, to shed new light on the controversial far-infrared opacity sources in Titan’s troposphere. Although a global cloud of large CH₄ ice particles around an altitude of 30 km, together with an increase in tropospheric haze opacity with respect to the stratosphere, can fit nadir and limb spectra well, this cloud does not seem consistent with shortwave measurements of Titan. Instead, the N₂-CH₄ collision-induced absorption coefficients are probably underestimated by at least 50% for low temperatures.     

Optical constants of Titan tholins at mid-infrared wavelengths (2.5–25 μm) and the possible chemical nature of Titan’s haze particles

Complex organic materials may exist as haze layers in the atmosphere of Titan and as dark coloring agents on icy satellite surfaces. Laboratory measurements of optical constants of plausible complex organic materials are necessary for quantitative evaluation from remote sensing observations, and to document the existence of complex organic materials in the extraterrestrial environments. The recent Cassini VIMS and CIRS observations provide new constraints on Titan’s haze properties in the mid-infrared wavelength region. Here, we present the optical constants (2.5–25 μm) of Titan tholins generated with cold plasma irradiation of a N₂/CH₄ (90/10) gas mixture at pressures of 0.26 mbar, 1.6 mbar, and 23 mbar. Our new optical constants of three types of Titan tholins suggest that no single Titan tholin in this study fulfills all the observational constraints of the Titan haze material. The discrepancy remains a challenge for future modeling and laboratory efforts that aim toward a better understanding of Titan’s haze material.     

Analytic theory of Titan’s Schumann resonance: Constraints on ionospheric conductivity and buried water ocean

This study presents an approximate model for the atypical Schumann resonance in Titan’s atmosphere that accounts for the observations of electromagnetic waves and the measurements of atmospheric conductivity performed with the Huygens Atmospheric Structure and Permittivity, Wave and Altimetry (HASI–PWA) instrumentation during the descent of the Huygens Probe through Titan’s atmosphere in January 2005. After many years of thorough analyses of the collected data, several arguments enable us to claim that the Extremely Low Frequency (ELF) wave observed at around 36 Hz displays all the characteristics of the second harmonic of a Schumann resonance. On Earth, this phenomenon is well known to be triggered by lightning activity. Given the lack of evidence of any thunderstorm activity on Titan, we proposed in early works a model based on an alternative powering mechanism involving the electric current sheets induced in Titan’s ionosphere by the Saturn’s magnetospheric plasma flow. The present study is a further step in improving the initial model and corroborating our preliminary assessments. We first develop an analytic theory of the guided modes that appear to be the most suitable for sustaining Schumann resonances in Titan’s atmosphere. We then introduce the characteristics of the Huygens electric field measurements in the equations, in order to constrain the physical parameters of the resonating cavity. The latter is assumed to be made of different structures distributed between an upper boundary, presumably made of a succession of thin ionized layers of stratospheric aerosols spread up to 150 km and a lower quasi-perfect conductive surface hidden beneath the non-conductive ground. The inner reflecting boundary is proposed to be a buried water–ammonia ocean lying at a likely depth of 55–80 km below a dielectric icy crust. Such estimate is found to comply with models suggesting that the internal heat could be transferred upwards by thermal conduction of the crust, while convective processes cannot be ruled out.     

Optical constants of Titan’s stratospheric aerosols in the 70–1500 cm−1 spectral range constrained by Cassini/CIRS observations

We utilized aerosol extinction coefficient inferred from Cassini/CIRS spectra in the far and mid infrared region to derive the extinction cross-section near an altitude of 190 km at 15°S (from far-IR) and 20°S (from mid-IR). By comparing the extinction cross section that are derived from observations with theoretical calculations for a fractal aggregate of 3000 monomers, each having a radius of 0.05 μm, and a fractal dimension of 2, we are able to constrain the refractive index of Titan’s aerosol between 70 and 1500 cm^?1 (143 and 6.7 μm). As the real and imaginary parts of the refractive index are related by the Kramers–Kronig equation, we apply an iterative process to determine the optical constants in the thermal infrared. The resulting spectral dependence of the imaginary index displays several spectral signatures, some of which are also seen for some Titan’s aerosol analogues (tholins) produced in laboratory experiments. We find that Titan’s aerosols are less absorbent than tholins in the thermal infrared. The most prominent emission bands observed in the mid-infrared are due to CH bending vibrations in methyl and methylene groups. It appears that Titan’s aerosols predominantly display vibrations implying carbon and hydrogen atoms and perhaps marginally nitrogen. In the mid infrared, all the aerosol spectral signatures are observed at three additional latitudes (56°S, 5°N and 30°N) and in the 193–274 km altitude range, which implies that Titan’s aerosols exhibit the same chemical composition in all investigated latitude and altitude regions.     

Circulation of the Venus upper mesosphere/lower thermosphere: Doppler wind measurements from 2001–2009 inferior conjunction,sub-millimeter CO absorption line observations

Sub-millimeter ¹²CO (346 GHz) and ¹³CO (330 GHz) line absorptions, formed within the mesospheric to lower thermospheric altitude (70–120 km) region of the Venus atmosphere, have been mapped across the nightside disk of Venus during 2001–2009 inferior conjunctions, employing the James Clerk Maxwell Telescope (JCMT). Radiative transfer analysis of these thermal line absorptions supports temperature and CO mixing profile retrievals, as described in a companion paper (Clancy et al., 2012). Here, we consider the analysis of the sharp line absorption cores of these CO spectra in terms of accurate Doppler wind profile measurements at 95–115 km altitudes versus local time (～8 pm–4 am) and latitude (～60N–60S). These Doppler wind measurements support determinations of the nightside zonal and subsolar-to-antisolar (SSAS) circulation components over a variety of timescales. The average behavior fitted from 21 retrieved maps of ¹²CO Doppler winds (obtained over hourly, daily, weekly, and interannual intervals) indicates stronger average zonal (85 m/s retrograde) versus SSAS (65 m/s) circulation at the 1 μbar pressure (108–110 km altitude) level. However, the absolute and relative magnitudes of these circulation components exhibit extreme variability over daily to weekly timescales. Furthermore, the individual Doppler wind measurements within each nightside mapping observation generally show significant deviations (20–50 m/s, averaged over 5000 km horizontal scales) from the simple zonal/SSAS solution, with distinct local time and latitudinal characters that are also time variable. These large scale residual circulations contribute 30–70% of the observed nightside Doppler winds at any given time, and may be most responsible for global variations in nightside lower thermospheric trace composition and temperatures, as coincidentally retrieved CO abundance and temperature distributions do not correlate with solution retrograde zonal and SSAS winds (see companion paper, Clancy et al., 2012). Limited comparisons of these nightside submillimeter results with dayside infrared Doppler wind measurements suggest distinct dayside versus nightside circulations, in terms of zonal winds in particular. Combined ¹²CO and ¹³CO Doppler wind mapping observations obtained since 2004 indicate that the average zonal and SSAS wind components increase by 50–100% between altitudes of 100 and 115 km. If gravity waves originating from the cloud levels are responsible for the extension of zonal winds into the thermosphere (Alexander, M.J. [1992]. Geophys. Res. Lett. 19, 2207–2210), such waves deposit substantial momentum (i.e., break) in the lower nightside thermosphere.     

Constraints on Saturn’s tropospheric general circulation from Cassini ISS images

An automated cloud tracking algorithm is applied to Cassini Imaging Science Subsystem high-resolution apoapsis images of Saturn from 2005 and 2007 and moderate resolution images from 2011 and 2012 to define the near-global distribution of zonal winds and eddy momentum fluxes at the middle troposphere cloud level and in the upper troposphere haze. Improvements in the tracking algorithm combined with the greater feature contrast in the northern hemisphere during the approach to spring equinox allow for better rejection of erroneous wind vectors, a more objective assessment at any latitude of the quality of the mean zonal wind, and a population of winds comparable in size to that available for the much higher contrast atmosphere of Jupiter. Zonal winds at cloud level changed little between 2005 and 2007 at all latitudes sampled. Upper troposphere zonal winds derived from methane band images are ～10 m s^?1 weaker than cloud level winds in the cores of eastward jets and ～5 m s^?1 stronger on either side of the jet core, i.e., eastward jets appear to broaden with increasing altitude. In westward jet regions winds are approximately the same at both altitudes. Lateral eddy momentum fluxes are directed into eastward jet cores, including the strong equatorial jet, and away from westward jet cores and weaken with increasing altitude on the flanks of the eastward jets, consistent with the upward broadening of these jets. The conversion rate of eddy to mean zonal kinetic energy at the visible cloud level is larger in eastward jet regions (5.2 × 10^?5 m² s^?3) and smaller in westward jet regions (1.6 × 10^?5 m² s^?3) than the global mean value (4.1 × 10^?5 m² s^?3). Overall the results are consistent with theories that suggest that the jets and the overturning meridional circulation at cloud level on Saturn are maintained at least in part by eddies due to instabilities of the large-scale flow near and/or below the cloud level.     

The Huygens Probe Descent Trajectory Working Group: Organizational framework,goals, and implementation

Cassini/Huygens, a flagship mission to explore the rings, atmosphere, magnetic field, and moons that make up the Saturn system, is a joint endeavor of the National Aeronautics and Space Administration, the European Space Agency, and Agenzia Spaziale Italiana. Comprising two spacecraft—a Saturn orbiter built by NASA and a Titan entry/descent probe built by the European Space Agency—Cassini/Huygens was launched in October 1997. The Huygens probe parachuted to the surface of Titan in January 2005. During the descent, six science instruments provided in situ measurements of Titan's atmosphere, clouds, and winds, and photographed Titan's surface. To correctly interpret and correlate results from the probe science experiments, and to provide a reference set of data for ground-truth calibration of orbiter remote sensing measurements, an accurate reconstruction of the probe entry and descent trajectory and surface landing location is necessary. The Huygens Descent Trajectory Working Group was chartered in 1996 as a subgroup of the Huygens Science Working Team to develop and implement an organizational framework and retrieval methodologies for the probe descent trajectory reconstruction from the entry altitude of 1270 km to the surface using navigation data, and engineering and science data acquired by the instruments on the Huygens Probe. This paper presents an overview of the Descent Trajectory Working Group, including the history, rationale, goals and objectives, organizational framework, rules and procedures, and implementation.     

A tidal explanation for the Titan haze layers

Though Titan is in synchronous rotation around Saturn, it experiences gravitational tides as a consequence of its eccentric orbit. It is proposed that the vertical transport of aerosols by these tides produces the haze layers in Titan's upper atmosphere. Analysis shows that the zonal winds in Titan's superrotating atmosphere have a profound influence on which tidal components are effective in establishing the multiple detached-haze layers. If the Huygens Doppler winds are representative of the equatorial global superrotation, then the westward propagating s=2 mode is the responsible tidal component even though its forcing is significantly weaker than that of the s=0 and eastward s=2 components. The eastward s=2 tidal mode is eliminated by critical levels while the s=0 mode is viscously damped in the strong high altitude winds. At polar latitudes, however, the gravest s=0 mode is the one most likely to produce layering. It is also suggested that the atmospheric gravitational tides could be responsible for decelerating the superrotating atmosphere as seen in the Huygens Doppler wind velocity profile at about 80 km altitude.     

A method to determine the atmospheric temperature profile from in situ pressure data: Application to Titan

We have developed a new method to analyse in situ observations of atmospheric variables of state: the reconstruction of the vertical temperature profile from pressure measurements accompanied by rough knowledge of the atmospheric composition and the aerodynamical response properties of the descent vehicle. We can use the method to construct the temperature profile when no direct measurements are available, as well as to analyse the consistency between data from different instruments. We applied the method to the Huygens measurements of Titan's atmosphere, determining the aerodynamical drag properties from radar altimeter data. We discovered that the temperature profile computed in this manner differs from the profile from the temperature sensor (TEM) of the probe by up to 5% in the altitude range of 0–60 km, and up to 10% at higher altitudes due to increased noise. The method gives a tropopause altitude of about 50 km and a surface temperature of about 98 K, in contrast to the TEM temperature measurements. Our error analysis shows that these differences are caused by the known discrepancy in the Huygens altimeter data, with the estimates made by the reconstruction algorithm contributing only 1–2% of error.     

Dissipation of Titan's north polar cloud at northern spring equinox

Saturn's Moon Titan has a thick atmosphere with a meteorological cycle. We report on the evolution of the giant cloud system covering its north pole using observations acquired by the Visual and Infrared Mapping Spectrometer onboard the Cassini spacecraft. A radiative transfer model in spherical geometry shows that the clouds are found at an altitude between 30 and 65 km. We also show that the polar cloud system vanished progressively as Titan approached equinox in August 2009, revealing at optical wavelengths the underlying sea known as Kraken Mare. This decrease of activity suggests that the north-polar downwelling has begun to shut off. Such a scenario is compared with the Titan global circulation model of Rannou et al. (2006), which predicts a decrease of cloud coverage in northern latitudes at the same period of time.     

Heat balance in Titan's atmosphere

The recent measurements of the vertical distribution and optical properties of haze aerosols as well as of the absorption coefficients for methane at long paths and cold temperatures by the Huygens entry probe of Titan permit the computation of the solar heating rate on Titan with greater certainty than heretofore. We use the haze model derived from the Descent Imager/Spectral Radiometer (DISR) instrument on the Huygens probe [Tomasko, M.G., Doose, L., Engel, S., Dafoe, L.E., West, R., Lemmon, M., Karkoschka, E., See, C., 2008a. A model of Titan's aerosols based on measurements made inside the atmosphere. Planet. Space Sci., this issue, doi:10.1016/j.pss.2007.11.019] to evaluate the variation in solar heating rate with altitude and solar zenith angle in Titan's atmosphere. We find the disk-averaged solar energy deposition profile to be in remarkably good agreement with earlier estimates using very different aerosol distributions and optical properties. We also evaluated the radiative cooling rate using measurements of the thermal emission spectrum by the Cassini Composite Infrared Spectrometer (CIRS) around the latitude of the Huygens site. The thermal flux was calculated as a function of altitude using temperature, gas, and haze profiles derived from Huygens and Cassini/CIRS data. We find that the cooling rate profile is in good agreement with the solar heating profile averaged over the planet if the haze structure is assumed the same at all latitudes. We also computed the solar energy deposition profile at the 10°S latitude of the probe-landing site averaged over one Titan day. We find that some 80% of the sunlight that strikes the top of the atmosphere at this latitude is absorbed in all, with 60% of the incident solar energy absorbed below 150 km, 40% below 80 km, and 11% at the surface at the time of the Huygens landing near the beginning of summer in the southern hemisphere. We compare the radiative cooling rate with the solar heating rate near the Huygens landing site averaging over all longitudes. At this location, we find that the solar heating rate exceeds the radiative cooling rate by a maximum of 0.5 K/Titan day near 120 km altitude and decreases strongly above and below this altitude. Since there is no evidence that the temperature structure at this latitude is changing, the general circulation must redistribute this heat to higher latitudes.     

Direct wind measurements from November 2007 in Venus’ upper atmosphere using ground-based heterodyne spectroscopy of CO2 at 10 μm wavelength

Between November 23 and 28, 2007, the Cologne Tuneable Heterodyne Infrared Spectrometer THIS was installed at the McMath-Pierce Solar Telescope (Kitt Peak, Arizona, USA) to determine zonal wind velocities and to estimate the subsolar-to-antisolar flow. We investigate dynamics in the upper atmosphere of Venus by measuring the Doppler shift of fully-resolved non-LTE CO₂ emission lines at 959.3917 cm^?1 (10.423 μm), which probe a narrow altitude region in Venus’ atmosphere around 110 ± 10 km (～1 μbar). The results show no significant zonal wind velocity at the equator. An increase with latitude up to 43 ± 13 m/s at a latitude of 33°N was observed. This confirms the deduction of a minor influence of Venus superrotation at an altitude of 110 km from previous measurements in May 2007 (Sornig et al., 2008). The specific observing geometry enables estimating the maximum cross terminator velocity of the subsolar-to-antisolar flow at 72 ± 47 m/s.